The principal conventional process for the dry quenching of coke, which is almost six decades old, consists in passing an inert gas in a closed circuit through the incandescent coke discharged from the coke oven into a closed vessel in which heat exchange is effected between the gas and the coke. The closed path for the gas includes a heat exchanger which can be used to generate steam by indirect heat exchange between the hot gas and water. This process has been found to yield coke of excellent quality for blast-furnace use or the like.
The advantage of the dry-quenching process over wet quenching is primarily a consequence of the lesser degree of environmental pollution or disadvantage resulting from the closed gas cycle.
In wet-quenching processes in which the coke is quenched with the aid of jets of water, at least part of the sensible heat of the coke is transformed into thermal energy of evaporation to vaporize the water. The vapors which result from the wet quenching process tend to become atmospheric pollutants and, in addition, residual water, upon separation from the coke, must be treated before it is disposed of.
A further disadvantage of the wet treatment process is that it is accompanied by a significant loss of water by evaporation and by absorption by the coke. The coke quality is reduced in that its mechanical strength is diminished so that it is less able to withstand the rigors of blast-furnace operation. Finally, the wet process gives rise to extremely moist coke fines which must be dried prior to use and agglomeration.
None of the aforementioned disadvantages characterizes the dry-quenching process and hence the dry-quenching process has found application whenever high quality cokes are to be produced.
However, the dry-quenching process has a significant disadvantage in that the gases which traverse the coke mass are charged with considerable quantities of abrasive powders and with coke fines which cause corrosion and erosion problems in the ducts, in the steam generator, and in the blowers used to circulate the gas.
Furthermore, because of the irregular introduction of coke into the cooling silo and discharge from the latter, the production of the steam is also irregular. In other words, greater or lesser quantities of steam are produced to the detriment of effective utilization of the recovered heat.
To overcome at least in part this drawback, it has been proposed to provide a bypass for the heat exchange circulation path which permits mixing of a cold gas with the hot recirculating gas so as to maintain the temperature of the gas more constant as it traverses the steam generator or heat exchanger in which the gas is cooled and the heat carried by the gas is transformed into steam. This is not, however, fully satisfactory.